In an MRI apparatus (or an MR scanner), an examination object, usually a patient, is exposed within the examination space of the MRI apparatus to a uniform main magnetic field (B0 field) so that the magnetic moments of the nuclei within the examination object tend to rotate around the axis of the applied B0 field (Larmor precession) with a certain net magnetization of all nuclei parallel to the B0 field. The rate of precession is called Larmor frequency which is dependent on the specific physical characteristics of the involved nuclei and the strength of the applied B0 field.
By transmitting an RF excitation pulse (B1 field) which is orthogonal to the B0 field, generated by means of an RF transmit antenna or coil, and matching the Larmor frequency of the nuclei of interest, the spins of the nuclei are excited and brought into phase, and a deflection of their net magnetization from the direction of the B0 field is obtained, so that a transversal component in relation to the longitudinal component of the net magnetization is generated.
After termination of the RF excitation pulse, the MR relaxation processes of the longitudinal and transversal components of the net magnetization begin, until the net magnetization has returned to its equilibrium state. MR relaxation signals which are emitted by the relaxation processes, are detected by means of an RF/MR receive antenna or coil. The received MR signals which are time-based amplitude signals, are Fourier transformed to frequency-based MR spectrum signals and processed for generating an MR image of the nuclei of interest within an examination object.
The above RF (transmit and/or receive) antennas can be provided both in the form of so-called body coils (also called whole body coils) which are fixedly mounted within an examination space of an MRI system for imaging a whole examination object, and as so-called surface or local coils which are arranged directly on or around a local zone or area to be examined and which are constructed e.g. in the form of flexible pads or sleeves or cages like head coils.
Further, interventional instruments or medical devices are frequently used during the examination or treatment of an examination object and especially of a local zone or area thereof. Such instruments or devices are for example catheters, biopsy needles, pointers and other which are used for example for biopsies, thermal ablations, brachytherapy, slice selection and other invasive or non-invasive purposes.
Both the above mentioned interventional and non-interventional instruments are usually connected by means of an RF transmission line (especially in the form of an electrically shielded line like a coaxial line) or a cable (which is usually unshielded) comprising at least one electrical conductor like a wire or a strip lines (which are applied onto a carrier like a printed circuit board), with related RF transmitter units, MR receiver units, or other signal processing or control units. It is generally known, that all these RF transmission lines and cables are subject to heating when they are guided through the examination space of an MR imaging apparatus and are exposed during the examination or imaging of an examination object to the transmitted RF excitation field. Such a heating is especially caused by resonant RF common mode currents which are induced in the RF transmission line or cable by the RF excitation field.
Several solutions (like e.g. a mechanical segmentation of the conductors) have been proposed in order to suppress standing waves and to avoid or minimize such a heating and to provide a so called “MR-safe” connection line.
U.S. Pat. No. 5,332,990 discloses a high-frequency safety fuse which comprises a fusible conductor which in its cross-section has a dimension in at least one direction which is substantially twice the penetration depth of the RF current. This fuse is disclosed to be suitable for use in an MR tomographic apparatus for the purpose of protecting the patient against topically excessive RF loads in a local coil circuit.